きのう ＿＿＿ ＿＿＿ ＿★＿ ＿＿＿ おもしろかったです。

Translation: The movie I watched yesterday was very interesting. - Sentence assembly: The correct order is 'きのう [① 見た] [② 映画] [★ ③ は] [④ とても] おもしろかったです。' - Grammatical point: Noun-modifying clause 'きのう見た' modifies '映画'. Then 'は' marks the entire modified noun phrase as the topic, followed by the adverb 'とても' and adjective 'おもしろかったです'. - The third slot (★) is filled by 'は' (Choice 3). - Therefore, the answer is 'は'.